The present invention relates to a toner for the development of an electrostatic image for use in electrophotographic process copying machines and printers. More particularly, the present invention relates to a toner for the development of an electrostatic image having excellent fixability, offset resistance and blocking resistance which can provide an image with a good OHP transparency.
A toner for the development of an electrostatic image which has heretofore been widely used in electrophotography is prepared by a process which comprises melt-kneading a mixture of a styrene-acrylate copolymer, a colorant such as carbon black and pigment, a charge control agent and/or a magnetic material through an extruder, crushing the material, and then classifying the powder. However, the conventional toner obtained by the foregoing melt-kneading/crushing process is disadvantageous in that the controllability of the particle diameter of the toner is limited, making it difficult to prepare a toner substantially having an average particle diameter of not more than 10 xcexcm, particularly not more than 8 xcexcm in a good yield. Thus, the conventional toner cannot be considered good enough to realize a high resolution which will be required in the future electrophotography.
Further, from the standpoint of reduction of energy required, it has been desired to provide a toner having a good low temperature fixability. To this end, an approach involving the blend of a low softening wax in a toner during kneading has been proposed. In the kneading/crushing process, however, the amount of such a wax to be blended in 100 parts of the resin is limited to about 4 to 5parts. Thus, toners having a sufficient low temperature fixability cannot be obtained.
In an attempt to eliminate these difficulties, JP-A-60-220358 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) and JP-A-60-225170 propose a process for the preparation of a particulate toner which involves emulsion polymerization in the presence of a colorant, followed by salting out of the emulsion polymer solution under predetermined conditions. Further, JP-A-2-616650 proposes a process which involves the mixing of an emulsion polymer solution with a dispersion of colorant, followed by coagulation of particles by salting out. According to these processes, the particles obtained at the agglomeration step have a particle diameter of not more than 25 xcexcm and thus can provide a particulate toner without passing through crushing step. However, these processes leave something to be desired in the control over the particle diameter distribution. Thus, a classification step is indispensable. Further, these processes are disadvantageous in that the yield of a toner having a desired particle diameter is poor.
In an attempt to overcome the forgoing difficulty in controlling the particle diameter and particle diameter distribution and hence realize a high resolution, JP-A-63-186253 proposes a process for the preparation of a toner involving emulsion polymerization/two-stage agglomeration process. However, this process, too, is limited in the amount of a wax to be introduced into the agglomeration step. Thus, this process leaves something to be desired in the improvement in low temperature fixability.
A process disclosed in JP-A-6-329947 involves the addition of an organic solvent infinitely soluble in water at the same time with a flocculating agent at the agglomeration step which allows the formation of aggregates having a narrow particle diameter distribution. However, this process is disadvantageous in that it has many factors to be controlled and hence shows a poor reproducibility. This process is also disadvantageous in that it gives a great burden of disposal of waste water.
It is therefore an object of the present invention to overcome the difficulties of the conventional toner for the development of an electrostatic image and hence provide a process for the preparation of a novel toner satisfying a high resolution, a low temperature fixability and a high offset resistance at a low cost.
The object of the present invention will become more apparent from the following detailed description and examples.
The inventors made extensive studies of the foregoing objects. As a result, it was found that the use of primary polymer particles obtained by the emulsion polymerization of monomers in the presence of a wax emulsion as a seed makes it possible to solve the foregoing problems. The present invention has thus been worked out.
The essence of the present invention lies in a toner for the development of an electrostatic image comprising an agglomerate of particles containing at least primary polymer particles, wherein said primary polymer particles substantially comprise a wax encapsulated therein and a process for the preparation of a toner for the development of an electrostatic image which comprises a first step of subjecting a monomer mixture containing a monomer having an acidic or basic polar group to seeded emulsion polymerization in the presence of a particulate wax as a seed, a second step of mixing a dispersion of primary polymer particles thus obtained with a dispersion of particulate colorant, and a third step of causing the mixture of dispersions to be agglomerated to form aggregates.
The present invention will be further described hereinafter.
As the wax to be used as a seed there may be any known wax. Examples of such a wax include olefinic wax such as low molecular weight polyethylene, low molecular weight polypropylene and polyethylene copolymer, paraffin wax, ester-based wax having long-chain aliphatic group such as behenyl behenate, montanic acid ester and stearyl stearate, vegetable wax such as hydrogenated castor oil carbanauba wax, ketone having long-chain alkyl group such as distearyl ketone, silicone having alkyl side group, higher fatty acid such as stearic acid, long-chain fatty acid alcohol, long-chain fatty acid-based polyvalent alcohol such as pentaerythritol, partial esterification product thereof, and higher fatty acid amide such as oleic acid amide and stearic acid amide.
Among these waxes, those having a melting point of not higher than 100xc2x0 C., preferably from 40xc2x0 C. to 90xc2x0 C., particularly from 50xc2x0 C. to 80xc2x0 C., are preferably used to improve the fixability of the toner. If the melting point of the wax exceeds 100xc2x0 C., the resulting effect of lowering the fixing temperature of the toner is poor.
The particulate wax employable herein can be obtained by the emulsification of the foregoing wax in the presence of at least an emulsifying agent selected from the group consisting of known cationic surface active agents, anionic surface active agents and nonionic surface active agents. Two or more of these surface active agents may be used in combination.
Specific examples of the cationic surface active agent employable herein include dodecyl ammonium chloride, dodecyl ammonium bromide, dodecyl trimethyl ammonium bromide, dodecyl pyridinium chloride, dodecyl pyridinium bromide, and hexadecyl trimethyl ammonium bromide.
Specific examples of the anionic surface active agent employable herein include fatty acid soap such as sodium stearate and sodium dodecanate, sodium dodecylsulfate, sodium dodecylbenzene sulfonate, and lauryl sodium sulfate.
Specific examples of the nonionic surface active agent employable herein include dodecyl polyoxyethylene ether, hexadecyl polyoxyethylene ether, nonylphenyl polyoxyethylene ether, lauryl polyoxyethylene ether, sorbitan mono-oleate polyoxyethylene ether, and monodecanoylsuccrose.
In the present invention, these waxes are dispersed in the presence of an emulsifying agent to produce an emulsion which is then used for the seeded polymerization of resin. The average particle diameter of the wax emulsion is preferably from 0.01 xcexcm to 3 xcexcm, more preferably from 0.03 xcexcm to 1 xcexcm, particularly from 0.05 to 0.8 xcexcm. For the measurement of average particle diameter, Microtrack UPA produced by NIKKISO CO., LTD. may be used. If the average particle diameter of the wax emulsion exceeds 3 xcexcm, the polymer particles obtained by seeded polymerization have too large an average particle diameter to provide a toner which can give a high resolution. On the contrary, if the average particle diameter of the emulsion falls below 0.01 xcexcm, the primary polymer particles obtained by seeded polymerization have too low a wax content to sufficiently exert the effect of wax.
In order to effect seeded emulsion polymerization in the presence of the wax emulsion, a monomer having a polar group (monomer having an acidic or basic functional group) and other monomers are successively added to cause polymerization in the emulsion particle containing wax. During this procedure, these monomers may be separately added. Alternatively, a plurality of monomers may be previously mixed before added. Further, the composition of monomers to be added may be changed during addition. Moreover, these monomers may be added as they are or in the form of emulsion obtained by mixing with water and a surface active agent. As such a surface active agent there may be used one or more of the previously exemplified surface active agents.
During the progress of seeded emulsion polymerization, an emulsifying agent may be added to the wax emulsion in a predetermined amount. The polymerization initiator may be added before, at the same time with or after the addition of the monomers. These addition methods may be employed in combination.
Examples of the monomer having an acidic polar group employable herein include monomers having carboxylic group such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and cinnamic acid, and monomers having sulfonic acid group such as sulfonated styrene.
Examples of the monomer having a basic polar group include aminostyrene and a quaternary ammonium salt thereof, monomers containing nitrogen-containing heterocycles such as vinylpyridine and vinylpyrrolidone, (meth)acrylic acid esters having amino group such as dimethylaminoethyl acrylate and diethylaminoethyl methacrylate, (meth) acrylic acid esters having ammonium salt obtained by quaterizing these amino groups, acrylamide, N-propylacrylamide, N, N-dimethylacrylamide, N, N-dipropylacrylamide, N, N-dibutylacrylamide.
Examples of the other comonomers employable herein include styrenes such as styrene, methylstyrene, chlorostyrene, dichlorostyrene, p-tert-butylstyrene, p-n-butylstyrene and p-n-nonylstyrene, and (meth) acrylic acid esters such as methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, hydroxyethyl acrylate, ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, hydroxyethyl methacrylate and ethylhexyl methacrylate. Particularly preferred among these comonomers are styrene, butyl acrylate, etc.
These monomers may be used singly or in admixture. These monomers are preferably added such that the resulting polymer exhibits a glass transition temperature of from 40xc2x0 C. to 80xc2x0 C. If the glass transition temperature of the polymer exceeds 80xc2x0 C., the resulting toner exhibits too high a fixing temperature. Further, the resulting OHP transparency can be likely deteriorated. On the contrary, if the glass transition temperature of the polymer falls below 40xc2x0 C., the storage stability of the resulting toner is deteriorated to an extent such that problems can occur. As the monomer having an acidic polar group employable herein there is preferably used acrylic acid. As the other monomers employable herein there are preferably used styrene, acrylic acid ester and methacrylic acid ester.
Examples of the polymerization initiator employable herein include persulfates such as potassium persulfate, sodium persulfate and ammoniumpersulfate, redox initiators obtained by combining these persulfates as one component with reducing agents such as acidic sodium sulfite, initiators such as hydrogen peroxide, benzoyl peroxide, t-butyl hydroperoxide and cumene hydroperoxide, redox initiators obtained by combining these initiators as one component with reducing agents such as ferrous salt, 4, 4xe2x80x2-azobiscyanovaleric acid, and 2, 2xe2x80x2-azobis-isobutylonitrile. These polymerization initiators may be added before, at the same time with or after the addition of the monomers. These addition methods may be employed in combination.
In the present invention, any known chain transfer agent may be used as necessary. Specific examples of such a chain transfer agent include t-dodecyl mercaptan, 2-mercaptoethanol, diisopropyl xanthogen, carbon tetrachloride, and trichlorobromomethane. These chain transfer agents may be used singly or in combination. These chain transfer agents may be used in an amount of from 0 to 5% by weight based on the weight of the polymerizable monomers used.
The dispersion of primary polymer particles obtained at the first step contains primary polymer particles substantially having a wax encapsulated therein. Referring to the morphology of the primary polymer particles, they may be of core-shell type, phase separation type, occlusion type or the like. Alternatively, the primary polymer particles may be in the form of mixture of these morphologies. A particularly preferred morphology is core-shell type. The morphology of the primary polymer particles can be confirmed by observing a section of the particle under an electron microscope. The wax is normally used in an amount of from 1 to 40 parts by weight, preferably from 2 to 35 parts by weight, more preferably from 5 to 30 parts by weight based on 100 parts by weight of the binder resin used. Components other than wax such as pigment and charge control agent may be further used as a seed so far as they don""t depart from the scope of the present invention.
The average particle diameter of the primary polymer particles is normally from 0.05 xcexcm to 3 xcexcm, preferably from 0.1 xcexcm to 1 xcexcm, more preferably from 0.1 xcexcm to 0.5 xcexcm. If the average particle diameter of the primary polymer particles falls below 0.05 xcexcm, the agglomeration rate can be hardly controlled to disadvantage. On the contrary, if the average particle diameter of the primary polymer particles exceeds 3 xcexcm, the particle diameter of the particulate toner obtained by agglomeration is too great to provide a toner having a high resolution.
At the second step, the dispersion of primary polymer particles and the dispersion of colorant are mixed. Preferably, the dispersion of primary polymer particles is mixed with the dispersion of colorant, followed by the addition of an electrolyte in a predetermined amount.
As the colorant employable herein there may be used any of inorganic or organic pigments and organic dyes, in combination as necessary.
Specific examples of such a colorant include known dyes and pigments such as carbon black, aniline blue, phthalocyanine blue, phthalocyanine green, hansa yellow, rhodamine dye or pigment, chrome yellow, quinacridone, benzidine yellow, rose bengal, triallylmethane dye, monoazo dye or pigment, disazo dye or pigment, and condensed azo dye or pigment. These dyes or pigments may be used singly or in admixture. If the toner of the present invention is a full-color toner, benzidine yellow, monoazo dye or pigment or condensed azo dye or pigment is preferably used as a yellow dye or pigment, quinacridone dye or pigment or monoazo dye or pigment is preferably used as a magenta dye or pigment, and phthalocyanine blue is preferably used as a cyan dye or pigment.
The colorant is normally used in an amount of from 3 to 20 parts by weight based on 100 parts by weight of the binder resin used. The colorant, too, is used in the form of emulsion obtained by emulsifying in water in the presence of the foregoing known emulsifying agent. The average particle diameter of the colorant employable herein is preferably not more than 3 xcexcm. If the average particle diameter of the colorant is not less than 3 xcexcm, the distribution of particle diameter of agglomerated particles is deteriorated to disadvantage.
As the electrolyte to be used at the second step of the present invention there may be used any of organic salts and inorganic salts. Preferably, a monovalent or higher metal salt is used. Specific examples of such a salt include NaCl, KCl, LiCl, Na2SO4, K2SO4, Li2SO4, MgCl2, CaCl2, MgSO4, CaSO4, ZnSO4, Al2(SO4)3, and Fe2(SO4)3.
The amount of the electrolyte to be added may change depending on its kind. In practice, however, the electrolyte is used in an amount of from 0.1 to 50 parts by weight, preferably from 0.2 to 40 parts by weight, more preferably from 0.3 to 30 parts by weight based on 100 parts by weight of the solid content of polymer used. If the amount of the electrolyte to be added falls below 0.1 part by weight, the agglomeration reaction proceeds so slowly that fine particles having a diameter of not more than 1 xcexcm are left behind after agglomeration reaction or the average particle diameter of the aggregates thus obtained is not more than 3 xcexcm. Such aggregates are not appropriate as toner. On the contrary, if the amount of the electrolyte to be added exceeds 50 parts by weight, the agglomeration reaction proceeds too rapidly to control. The resulting aggregates contain coarse particles having a particle diameter of not less than 25 xcexcm or have an irregular amorphous form.
During the addition of the electrolyte, the temperature of the mixed dispersion is preferably kept to a range of not higher than 40xc2x0 C., more preferably not higher than 30xc2x0 C., even more preferably not higher than 20xc2x0 C. If the temperature of the mixed dispersion exceeds 40xc2x0 C. during the addition of the electrolyte, rapid agglomeration occurs, making it difficult to control the particle diameter or giving particles having a low bulk density.
The average particle diameter of the mixed dispersion obtained by the addition of the electrolyte is normally not more than 3 xcexcm, preferably not more than 2 xcexcm, more preferably not more than 1 xcexcm. If the average particle diameter of the mixed dispersion exceeds 3 xcexcm, the aggregates obtained at the subsequent step have a grape-like form, providing a toner having too low a strength.
Further, a particulate wax may be present at the second step to produce a mixture of particles as necessary. The particulate wax employable herein may be the same as or different from that used in the seeded polymerization mentioned above.
At the third step of the present invention, the mixed dispersion which has been obtained up to the second step is heated with stirring to produce aggregates. The stirring of the mixed dispersion may be effected in a reaction vessel equipped with a known agitator such as paddle agitator, anchor agitator, three-plate backward agitator and maxblend agitator or by means of a homogenizer, homomixer, Henschel mixer or the like.
At the third step it is preferable that a particulate charge control agent is added, because of a good triboelectricity and a good triboelectricity stability.
As the charge control agent there may be used any known charge control agents, singly or in combination. Taking into account the adaptability to color toner (charge control agent itself is colorless or has a light color and hence doesn""t impair the color tone of the toner), a quaternary ammonium salt compound is preferably used as a positively-charging charge control agent and a metal salt or metal complex of salicylic acid or alkylsalicylic acid with chromium, zinc or aluminum, a metal salt or metal complex of benzylic acid, amide compound, phenol compound, naphthol compound, phenolamide compound, etc. are preferably used as a negatively-charging charge control agent. The amount of the charge control agent to be used may be determined by the determined chargeability. In practice, however, it is normally from 0.01 to 10 parts by weight, preferably from 0.1 to 10 parts by weight based on 100 parts by weight of the binder resin used.
The growth of particle diameter by the agglomeration reaction at the third step proceeds until particles having a size substantially the same as that of the particulate toner are obtained. By controlling the pH value and temperature of the dispersion, it is made relatively easy to control the particle diameter of aggregates.
The pH value of the dispersion at the third step changes with the kind and amount of the emulsifying agent used and the desired particle diameter of the toner and thus cannot be unequivocally defined. In practice, however, if an anionic surface active agent is mainly used, the pH value of the dispersion is normally from 2 to 6. If a cationic surface active agent is mainly used, the pH value of the dispersion is normally from 8 to 12.
In the present invention, it is preferred that the mixing at the second step is effected at a temperature of not higher than 40xc2x0 C. and the agglomeration at the third step is effected at a temperature of from not lower than 40xc2x0 C. to not higher than the glass transition temperature (Tg) of the polymer plus 20xc2x0 C.
The reaction temperature is preferably the glass transition temperature (abbreviated as xe2x80x9cTgxe2x80x9d) of the resin plus 20xc2x0 C. The glass transition temperature of the resin can be measured by means of a differential scanning calorimeter (DSC). The reaction temperature is more preferably from Tg to (Tg+10xc2x0 C.). If the reaction temperature exceeds (Tg+20xc2x0 C.), the particle diameter of aggregates can be hardly controlled to a desired range, making it easy to produce coarse particles.
Referring to the agglomeration reaction, the dispersion is kept at the desired temperature for at least 10 minutes, preferably not less than 20 minutes, to produce a particulate toner having a desired particle diameter. The dispersion may be heated to the desired temperature at a constant rate or stepwise.
Further, in order to enhance the stability of the aggregates having a toner size obtained at the third step, a step of causing the fusion of agglomerated particles to each other at a temperature of from (Tg+20xc2x0 C.) to (Tg+80xc2x0 C.) may be added. In general, the fusion of the particles to each other proceeds further during this step, making it possible to round the shape of the toner particles or control the shape of the toner particles as necessary. This step is normally effected for 1 hour to 24 hours, preferably from 2 hours to 10 hours.
In the preparation of the toner of the present invention, the substantial growth of the particle diameter of the aggregates to the final toner particle diameter is followed by the addition of the same or different kind of a binder resin emulsion that causes particles to be attached to the surface of the toner particles, making it possible to modify the properties of the toner in the vicinity of the surface of the aggregates. For example, by causing a resin having a high glass transition temperature to be attached to the surface of the toner particles, the storage stability of the aggregates can be enhanced. Further, by causing a charge control agent or a particulate resin containing a charge control agent to be attached to the surface of the toner particles, the triboelectricity of the toner can be improved.
The toner according to the present invention can be used with an additive such as fluidity improver as necessary. Specific examples of such a fluidity improver include hydrophobic silica powder, titanium oxide powder, aluminum oxide powder, and magnesium oxide powder. Such a fluidity improver is normally used in an amount of from 0.01 to 5 parts by weight, preferably from 0.1 to 3 parts by weight based on 100 parts by weight of the binder resin used.
Further, the toner according to the present invention may have an inorganic fine powder such as magnetite, ferrite, cerium oxide, strontium titanate and electrically conductive titania or a resistivity adjustor or lubricant such as styrene resin, acrylic resin, zinc stearate and lithium stearate incorporated therein as an internal or external additive. The amount of such an additive to be added may be properly predetermined depending on the desired properties. In practice, however, it is preferably from 0.05 to 10 parts by weight based on 100 parts by weight of the binder resin used.
The toner for the development of an electrostatic image of the present invention may be in the form of either two-component developer or non-magnetic one-component developer. In particular, in the form of non-magnetic one-component developer, the toner of the present invention shows a good triboelectricity and a good triboelectricity stability. The toner of the present invention, if used as a two-component developer, may have any known carrier such as magnetic material such as iron powder, magnetite powder, ferrite powder, material obtained by coating the surface of such a magnetic material with a resin and magnetic carrier incorporated therein. As the coating resin to be used in the resin-coated carrier there may be used styrene resin, acrylic resin, styrene-acryl copolymer resin, silicone resin, modified silicone resin, fluororesin or mixture thereof.
The present invention will be further described in the following examples, but the present invention should not be construed as being limited thereto.
The term xe2x80x9cpartsxe2x80x9d as used hereinafter is meant to indicate xe2x80x9cparts by weightxe2x80x9d. For the measurement of the average particle diameter and molecular weight of the polymer particles, the following methods were used. Average particle diameter: Microtrack UPA produced by NIKKISO CO., LTD. or Coulter Multisizer II produced by Coulter Inc. was used. Distribution of particle diameter: coefficient variation (CV %) measured by Coulter Multisizer II
CV =(SDxc3x97100)/{overscore (X)}(%)
SD: standard deviation
{overscore (X)}: diameter
Weight-average molecular weight: Gel permeation chromatography (GPC) was employed. (Solvent: THF; calibration curve: standard polystyrene)
The toner obtained was subjected to fixing test according to the following method.
A recording paper having an unfixed toner image supported thereon was prepared. The recording paper was carried into the fixing nip through a pair of heated rolls, the surface temperature of which was varied between 100xc2x0 C. and 190xc2x0 C. The recording paper discharged from the fixing nip was then observed for fixing conditions. The temperature range within which the heated rolls undergo no toner offset during fixing and the toner which has been fixed to the recording paper is sufficiently bonded to the recording paper is defined as fixing temperature range. Supposing that the lower limit of the fixing temperature at which no offset occurs is TL and the upper limit of the fixing temperature at which no offset occurs is TU, the value obtained by subtracting TL from TU is the width of fixing temperature. As the fixing machine there was used one described in the following method 1 or 2.
(Method 1) The heated rolls in the fixing machine has a releasing layer made of FEP (tetrafluoroethylene-hexafluoropropylene copolymer). For the evaluation of fixing temperature, the nip width is predetermined to 5 mm.
(Method 2) The heated rolls in the fixing machine has a releasing layer made of PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer). For the evaluation of fixing temperature, the nip width is predetermined to 4 mm.